Green’s Dictionary of Slang

1. anything that has been fraudulently substituted for something else, typically a racehorse or dog; also attrib.

1901		Sydney Sportsman (Surry Hills, NSW) 30 Oct. 4/2: [A] certain Hebrew gentleman [...] punted right royally on the alleged ‘ring in,’ which duly won.
1906		Sun. Times (Perth) 18 Mar. 1/1: The winning side had a ‘ring in’ score to use if necessary [but] fortunately a surplus of one run rendered this deception unnecessary.
1929		Morn. Bulletin (Rockhampton, Qld) 16 Jan. n.p.: ‘A ring-in! A blooming choke!’ [...] and similar cries greeted officals at the finish of the Great Handicap.
1935		Mirror (Perth) 21 Sept. 16/6: [headline] ‘The Mirror’ Condcucts Some Investigations on Currency ‘Ring-Ins’.
1941		Baker Popular Dict. Aus. Sl. 60: Ring-in, a horse or dog that is fraudulently entered in a contest under an assumed name and/or disguised.
1950		Barrier Miner (Broken Hill, NSW) 9 Sept. 4: The police suspect that a valuable racehorse stolen from a Flemington railway siding may be used as a ‘ring-in’ in the country.
1959		D. Niland Gold in the Streets (1966) 203: You weren’t Chris Cassimatis at all, but a pretty poor ring-in for Chris Cassimatis.
1966		J. Holledge Great Aust. Gamble 119: [T]he notorious days of crooked trotting practices — of ring-ins and nobbling, of non-triers and interference, of doping and bribery .
1977		(con. 1941) R. Beilby Gunner 136: The skinny bloke’s ours. The fat bludger’s a ring-in we picked up on the way.
1990		S. Geason Shaved Fish 6: It’s a better job than Fine Cotton, Ronny, but it’s still a ring-in.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 257: [T]he ring-in hay pirate was going to be no better than third or fourth favourite and therefore should be no less than threes or fours in the betting.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 43: A life of break-and-enters, bashings, shootings, ring-in rorts and standovers.
2003		McGill Reed Dict. of N.Z. Sl. 173: ring-in A substitute or somebody invited in to make up the numbers, often in a sporting contest. Originally ANZ early C20 for a horse illegally substituted in a race.
2013		T. Peacock More You Bet 26: In the event of a runner being disqualified [...] by being disclosed as a ‘ring-in’ (or substitute) [etc.].

2. (Aus.) a substitute, no sense of illegality implied.

1986		R.G. Barrett Godson 162: ‘You sure you’re in line to the Royal Family, mate? You could be a ring-in. A lousy bloody commoner’.
2010		A. Shand King of Thieves [ebook] Some ring-ins took the raiding party’s total complement to ten, enough to control the scene.
2018		C. Hammer Scrublands [ebook] .

3. a stacked deck of cards.

1953

Baker Aus. Speaks ‘Brought you a ring-in, skipper,’ says Robbie, smiling [...] ‘Good lad. Hiya, Martin. Welcome to the team’.